1. Field of the Invention
The present invention relates generally to a resolution conversion technique for use in a system capable of manipulating digital images represented by a plurality of original image pixels, and more specifically to a method, capable of being used in an electroreprographic machine, for deriving an intensity value of a selected output pixel in a bitmap, the bitmap including the selected output pixel mapped to a plurality of correlated, neighboring input pixels.
2. Description of the Prior Art
Image information, be it color or black and white, is commonly generated in a bitmap format at a particular scale, and resolution K.times.L.times.b, corresponding to a desired printer output, where K is a number of spots per unit of length in one dimension, L is a number of spots per unit length in the other dimension, and b is the depth of each pixel, in number of levels. This bitmap is present for every color separation of the output device, i.e., 4 bitmaps for a 4-color output device, 3 for a 3-color, 2 for a 2-color and 1 for a black and white output device. In a common example of a black and white output, image data comprising a bitmap to be printed is provided to a printer suitable for printing at 300 spots per inch (spi) in both dimensions, at a one bit depth giving 2 levels. Many considerations drive this single selection of resolution, including the desirability of providing only a limited number of fonts (alphanumeric bitmaps) so as to use only a limited amount of storage space. Common software packages available on personal computers or for operation of input scanners for document creation also usually provide only a single resolution output.
Increasingly, the resolution available from printers varies over a wider range of choices. Printer resolutions are available over a range, for example, from less than 200 spi to more than 600 spi. Resolutions vary for a number of reasons, generally related to the quality of the output image. Simply printing a 300 spi bitmap at 400 spi or 600 spi is undesirable however, since the image will be reduced substantially in size on the output page or display. On the other hand, printing a 400 spi bitmap at 300 spi is undesirable since the image is simply enlarged, and commonly is visibly distorted at the lesser resolution. It would be highly desirable to provide the capability of printing any image at any resolution, while selecting the output size.
A system for manipulating binary digital images represented by a plurality of original image pixels is disclosed in U.S. patent application Ser. No. 07/513,415, entitled "Bit-Map Image Resolution Converter" ("Resolution Converter Application"), filed Apr. 23, 1990, the relevant portions of which are incorporated herein by reference. The Resolution Converter Application contemplates a method of magnifying, by a predetermined magnification factor (n), the original image pixels in two dimensions. The method includes the steps of selecting an original image pixel, as well as determining the binary state of both the selected original image pixel and all immediately surrounding original image pixels. Additionally, the selected original image pixel is expanded into an n.times.n array of magnified image pixels to represent the magnification of the selected original image pixel. Finally, a binary state is assigned to each pixel in the array of magnified image pixels according to the pattern of binary signals previously determined for the selected original image pixel and all immediately surrounding original image pixels.
In the preferred embodiment of the Resolution Converter Application, the assignment of the binary states to the pixels in the array of magnified image pixels is made according to a set of state determination rules. While the system of the Resolution Converter Application is well suited for performing resolution conversion under certain conditions, it is limited in its application. In particular, this system is only intended for magnifying a stored binary image by an integral value. Additionally, the state determination rules were apparently generated on an iterative basis rather than in accordance with a generic probabilistic function. It would be highly desirable to provide a technique in which resolution conversion is achieved in accordance with a mathematical function instead of a set of rules, and a stored image can be magnified or reduced by both integral and non-integral values.